The present invention relates to improvements in the so-called sinter bonding process for joining a plurality of green compacts together into a one piece sintered part.
The conventional brazing processes have generally made use of dimensional changes of green compacts due to sintering, viz., differences between the size of the green compacts and that of the sintered compacts at normal temperature.
Now assume that the dimensional changes of inner and outer parts, as defined in the appended claim, are designated as positive or negative when they expand or shrink.
To bond the inner part to the outer part, for example, the materials thereof have been chosen such that a dimensional change of the inner part is larger than that of the outer part, as is the case with the bonding of an inner part of Fe-7 to 15Cu (expansion) to an outer part of Fe-0.5 to 4Ni (contraction).
However, the conventional processes primarily rely upon mechanical joining which takes advantage of a so-called thermal-insert or shrink-fit mechanism, by which the integration of the inner and outer parts is not at all or or only partially achieved through the metal diffusion therebetween. This poses a problem in connection with the reliability of joining.
As a result of extensive studies made on the sintering process of various types of iron base sintered metals with the aid of a thermal dilatometer, however, it has been found that, a certain combination of the type and amount of additives gives rise to a reversal of the magnitude of dimensional changes of a sintered mass cooled down to normal temperature and a green compact exposed to a high-temperature region (in which the additives diffuse) during sintering, and that such a reversal phenomenon is observed only when there is a difference of 0.2% or higher in the carbon content between two green compacts if the amounts of other ingredients are the same. To avoid confusion, the dimensional change upon sintering and the dimensional change during sintering will hereinafter be referred to as the post change and the insintering change, respectively.